The present invention relates to a method of investment casting, and to a method of making an investment casting mould. The invention is particularly relevant to the casting of articles by directional solidification, and more particularly to the casting of single crystal articles.
In the investment casting process, or lost wax casting process as it is sometimes called, a wax pattern of an article, or component, is produced. The wax pattern is produced by injecting wax into an accurately formed die. The wax pattern is a replica of the article to be produced. Usually a number of wax patterns are assembled together on a wax gating tree to form a cluster or wax mould assembly. The wax mould assembly is immersed in a liquid ceramic slurry which quickly gels after draining. Strengthening refractory granules are sprinkled over the ceramic slurry covered wax mould assembly and the refractory granules bond to the slurry coating to produce a ceramic layer on the wax mould assembly. This process is repeated several times to produce many ceramic layers which have a total thickness of about 1/4 inch (6 mm) to 1/2 inch (12 mm) on the wax mould assembly. The wax is then melted out leaving a ceramic shell mould having an internal cavity identical in shape to that of the original wax mould assembly. This ceramic shell mould is called an investment casting mould. The mould is fired at a high temperature between 950.degree. C. and 1100.degree. C. to remove all traces of residual wax, and cure the ceramic shell mould. The ceramic shell mould is then transferred to a casting furnace, which may be operated at either vacuum conditions or at atmospheric conditions. A charge of molten metal is poured into the ceramic shell mould and the mould is allowed to cool to room temperature, after which the ceramic shell mould is removed leaving the cast article or articles. The ceramic shell mould may be cooled by applying a temperature gradient across the ceramic shell mould to directionally solidify the metal in order to produce columnar grains, or single crystals in the finished article or articles.
It is also known to produce resin patterns using stereolithography, rather than making wax patterns in a die. The advantage of using stereolithography is that it enables the patterns to be produced quickly for development purposes. The resin patterns are produced by directing a beam of focused radiation into a bath of liquid resin which is locally cured and solidified by the radiation. The beam of radiation is moved under computerised control to produce a resin pattern of the article to be produced. The resin pattern is then coated with ceramic slurry as discussed above to produce the ceramic shell mould. However, for production purposes resin patterns are not smooth enough for production quality articles and stereolithographic production of resin patterns is slow and expensive compared to the production of wax patterns by wax injection into a die.
The immersing, or dipping, of the wax mould assembly in the ceramic slurry is a relatively uncontrolled process. The build up of ceramic material is governed by the adhesion of the ceramic material onto the wax mould assembly. Random features such as drips and runs are common. In particular, the ceramic shell is thicker on concave external surfaces than on convex external surfaces of the wax mould assembly. In general the article features are blurred, sharp edges are blunted, fillet radii are enlarged, surfaces are smoothed and bridges may form between completely separate areas of the wax mould assembly. The thickness and external shape of the ceramic shell mould control the heat transfer out of the molten metal during the casting process.
It is necessary to have a mathematical description of the external surface of the ceramic shell mould. This description may be derived by running a mathematical model that simulates the build-up of ceramic on the article, for use in process models of the investment casting process to produce defect free cast articles.
There is currently no mathematical model of the external surface of the ceramic shell mould which simulates variation in the ceramic shell thickness with variations in the curvature of the external surface of the article. Furthermore there is currently no mathematical model of the external surface of the ceramic shell mould which simulates bridging between completely separate areas of the ceramic shell mould.